The universal mobile telecommunication system (UMTS) terrestrial radio access (UTRA) uses code division multiple access (CDMA) as its multiple access technique. On the uplink (mobile terminal to base station direction), non-orthogonal codes are used in combination with power control. However, because the codes are not orthogonal, the capacity of the uplink is limited by multiple access interference. The UTRA specification provides for the optional use of short codes to allow the use of various receiver techniques in the base station which rely on the fact that the multiple access interference is not noise but is, in fact, other signals. The receiver techniques which operate in this way are generically known as interference cancellation and joint detection.
One implementation of interference cancellation operates by first demodulating the data on all of the signals directed to the base station to form estimates of the data. Knowledge of these estimates of the data along with channel estimates allows the generation of delayed approximate replicas of the received signal from each of the mobile terminals. For each wanted signal, the replicas for the other signals are summed together and subtracted from a delayed version of the received composite signal. Thus, at this stage, the interference has been approximately cancelled for that signal. When demodulation (including despreading) is performed, the bit error rate (BER) should be reduced. The whole process can be repeated several times, each time using the improved estimates of the received data to construct the approximate replicas.
One implementation of joint detection operates by treating the sum of the signals as a composite signal having travelled over a path with components relating to the individual signal components. This path is then linearly or non-linearly equalised in order to permit demodulation of all of the data over all of the signals.
In both interference cancellation and joint detection techniques, it is necessary to have knowledge of the bit rates (and, therefore, the spreading factors) for each of the received signals. In UTRA frequency division duplex (FDD), the signal format consists of frames of 10 ms duration. There are two channels for each signal, namely, the dedicated physical control channel (DPCCH) and the dedicated physical data channel (DPDCH).
The DPCCH is a low power constant bit rate channel. It consists of 16 timeslots each comprising pilot symbols, forward error correction (FEC) encoded transport format indicator (TFI) data and transmit power control (TPC) data. The DPDCH consists of time interleaved, FEC encoded data. It has a bit rate which may vary from one frame to the next, the bit rate of which is carried by the TFI data in the DPCCH of the same frame. On the uplink, in a single spreading code transmission, the DPDCH is first spread to become the inphase (I) channel and the DPCCH is spread to become the quadrature (Q) channel. Overall scrambling is then applied to the combined signal.
The TFI data is spread out across the frame and cannot be reliably decoded until the whole of the current frame has been received. Thus the bit rate information, for each of the signals, is unavailable until the whole of the current frame has been received. This causes two problems:—
First, the reason for applying interference cancellation or joint detection is to increase the system capacity by allowing the reception of signals at a lower signal to noise plus interference ratio than would be possible without using it. This means that before the applicant of interference cancellation, it may be impossible to demodulate the TFI bits, leading to a deadlock situation. This is true even though the DPCCH and DPDCH are transmitted on nominally orthogonal (I and Q) channels since multipath will seriously degrade this orthogonality and because the different signals will be received at the base station with arbitrary mutual carrier phase.
Secondly, power control information is generated by making signal to noise plus interference measurements on the DPCCH within the time period of the frame. Thus, if interference cancellation or joint detection cannot be applied until the end of the frame, these measurements will need to be based on the signal to noise plus interference (SNI) without the benefit of interference cancellation or joint detection. If the power control measurement threshold is based on an adequate SNI ratio at this stages then the resultant SNI ratio after the operation of interference cancellation or joint detection will be higher than necessary. On the other hand, attempting to base the power control measurements on there being an adequate SNI ratio after the operation of interference cancellation or joint is problematic because: a) the SNI ratio at the measurement stage will be very low—probably too low to measure, and b) it is not possible to predict, a priori, how effective the interference cancellation or joint detection will be in any given slot.
U.S. Pat. No. 5,151,919 (Ericsson) provides a subtractive CDMA demodulation system which optimally decodes a coded system embedded in many other overlapping signals making up a received composite signal. A radio receiver correlated a unique code corresponding o the desired signal to be decoded with the composite signal. WO96/24206 (Nokia) provides a CDMA system in which several users communicate simultaneously on the same frequency band, and in which each user has its own spreading code. For reception of signals, sigal correlators use synchronisation with waveforms of different types to aid decoding.